Maybe it's because D&D blurs the line between "human enemy" and "monster enemy". Orcs, ogres, gnolls, and other humanoids... people or monsters? It's a slippery slope from there. But once you've decided that "always Chaotic Evil humanoids" can be slain without mercy, why should that give the heroes license to do the same thing to human bandits?
Well, it shouldn't. Player characters who go around wantonly and injudiciously killing people, even criminal people, are Neutrally aligned at best. In D&D, that means paying attention to the alignment a player has chosen vs. the alignment the player is actually playing, and reacting accordingly. In a Star Wars, LotR, or similar sort of campaign setting with strict and universal morality, characters still playing in "D&D mode" (i.e. kill 'em all, take their stuff, and let the gods sort 'em to the right Outer Planes) are just plain doing it wrong.
*sigh* Now I'm bummed out. Where are all the heroes in RPGs, anyway?
On a lighter note, I just found out yesterday that www.garfield.com has the old U.S. Acres comic strip up as a webcomic! How cool is that? Orson, Roy, Wade, Booker & Sheldon, Bo & Lanolin... *sigh again* Happy memories!